1. Field of the Invention
This invention relates to the switching between an AC ringing voltage and a DC trip voltage which occurs at many telephone company central offices and more particularly to a circuit which performs such switching.
2. Description of the Prior Art
A typical telephone central office provides service to a large number of subscribers who are connected to the office by cable pairs. The central office usually includes one or more generators each associated with a predetermined number of the subscribers served by the office. Each generator provides an AC signal or voltage for ringing the subscriber telephones. That AC signal is superimposed on the central office battery to give rise to a composite signal having both an AC and a DC component.
When it is desired to ring the telephones of the subscribers served by that office, the composite signal from the associated generator is applied to the appropriate cable pairs. The composite signal will be referred to hereinafter as the "ringing signal or ringing voltage".
The subscriber telephones are usually rung in a pattern. The ringing signal is applied to the cable pairs for a predetermined interval, typically two seconds. If at the end of that interval the called subscribers have not answered or the calling parties have not abandoned the call, the ringing voltage is removed from the cable pairs and central office battery also known as the quiet interval tripping voltage is applied to the cable pairs for a predetermined interval, typically four seconds. This pattern of applying ringing voltage followed by tripping voltage to the cable pairs continues until either the called subscriber answers or the calling party abandons the call.
Telephone operating companies must then include at the central office, equipment which switches back and forth between the ringing voltage and the tripping voltage. Various devices and circuit designs have been used in the past to accomplish that switching. Examples of such devices and circuit designs are:
(i) switching devices using electromechanical relays or rotary cam and spring interrupters;
(ii) switching circuits using bipolar transistors; and
(iii) switching circuits using triacs and inverse parallel silicon controlled rectifiers (SCRs).
The switching devices which use electromechanical relays or rotary cam and spring interrupters may have the disadvantage of higher initial cost as compared to switching circuits which use solid state components. Those devices are unable to perform switching at the zero crossing of the ringing voltage. Such switching is desirable as it minimizes induced noise in adjacent central office cable pairs. Zero crossing is also important to avoid switching at the negative peak of the superimposed ringing and battery into a highly capacitive load such as tens to hundreds of subscriber cable pairs. Finally, such devices have a relatively short life due to progressive contact deterioration and eventual mechanical wearout. To limit the contact erosion of the switching device from highly reactive loads, the electromechanical devices must be oversized or multiple devices must be employed, each carrying a fraction of the total central office load.
The switching circuit designs which use bipolar transistors introduce undesirable distortion where ringback tone is superimposed on the ringing waveform. The superimposed ringback tone is the tone heard by the calling party during the interval when the subscriber's telephone is rung. The undesirable distortion is heard by the calling party as clicks and is due to the saturation voltage of the transistors. Those circuits which use triacs and inverse parallel SCRs also introduce distortion in the ringing waveform due to the voltage drop across the devices. In addition, care must be taken in designing circuits which use triacs and inverse parallel SCRs to insure proper device commutation.
As can be seen from the above discussion, it is desirable that a ringing switch use solid state rather than mechanical devices. It is further desirable that such a solid state switching circuit be easy to design, have true zero crossing switching and not introduce distortion where ringback tone is superimposed on the ringing waveform. None of the prior art switching units can meet all of these requirements. The solid state switch of the present invention does.